When inspecting a fire-hydrant, the main task of the inspector is to operate the hydrant to check that the water flow is adequate. After the test, the barrel of the hydrant is now full of water. The operator cannot just screw the port-cap back on, as that would prevent the water in the barrel from draining away.
So the operator has to waste time waiting for the barrel to drain, which can take twenty or thirty minutes, before he can screw the port-cap back on.
The invention is aimed at enabling the hydrant to drain by itself, so the operator can walk away from the hydrant immediately after the test.